1. Field of the Invention
The present invention relates to highly sensitive heat-sensitive recording materials having excellent thermal response.
2. Description of the Prior Art
A heat-sensitive recording material comprises a support having provided thereon a heat-sensitive recording layer composed mainly of an electron donative, generally colorless or light-colored dye precursor and an electron accepting color developer. Upon heating with a thermal head, a thermal pen, a laser light, etc., the dye precursor instantaneously reacts with the color developer to give recording images and, such a system is disclosed in Published Examined Japanese Patent Application KOKOKU Nos. 4160/68, 14039/70, etc. Such a heat-sensitive recording material has been utilized over a wide range of fields such as a measuring recorder, a facsimile machine, a printer, a terminal of a computer, a label, a passometer, etc. because recording can be easily made using a relatively simple device, maintenance is easy, noise is not produced, etc. Particularly in facsimile machines, a great demand for heat-sensitive type continues to expand and at the same time, its speed is getting higher and higher because of necessity for reducing transmission costs. In response to such a high speed facsimile, demand for the high speed heat-sensitive recording materials is increasing.
In high speed facsimile machines, a current is repeatedly applied to a thermal head in such a very short period of time as less than several miliseconds because a standard original of a A-4 size is received and transmitted in a time period of several to 20 seconds and, the thermal energy generated thereby is transmitted to a heat-sensitive recording sheet in which a reaction for forming images is performed.
In order to perform the reaction for forming images by the thermal energy transmitted in such a short period of time, it is required that the heat-sensitive recording material be excellent in thermal response. To enhance the thermal response or reactivity, compatibility of a color developer with a dye precursor should be improved. For this purpose, senstizers are employed depending upon necessity. The sensitizers have an action to accelerate a color-forming reaction, by dissolving or enveloping therein dye precursors and color developers present around them when the sensitizers themselves melt with the transferred thermal energy. For increasing the sensitivity of a heat-sensitive recording material, it is also one technique to improve thermal response of the sensitizers or compatibility of the sensitizers with the dye precursors or color developers.
As such a technique, there are disclosed method for incorporating waxes in Published Unexamined Japanese Patent Application KOKAI No. 19231/73; nitrogen-containing compounds, carboxylic acid esters, etc. in Published Unexamined Japanese Patent Application KOKAI Nos. 34842/74, 149353/75, 106746/77, 5636/78, etc.; naphthol derivatives in Published Unexamined Japanese Patent Application KOKAI Nos. 64593/82 and 87094/84; naphthoic acid derivatives in Published Unexamined Japanese Patent Application KOKAI Nos. 64592/82, 185187/82, 191089/82 and 110289/83; and benzoic acid ester derivatives in Published Unexamined Japanese Patent Application KOKAI Nos. 148688/82, 182483/82, 112788/83 and 162379/83.
However, the heat-sensitive recording materials prepared by these methods are still insufficient for color density and color sensitivity. The present inventors have thus made investigations on various sensitizers with an attempt to obtain highly sensitive heat-sensitive recording materials having superior thermal response.